Before the Iraq war was launched in March 2003 the world was given the impression by the US and Britain that the goal was to eradicate weapons of mass destruction. Recent comments by Tony Blair suggest, however, that regime change was the essential aim. He would have thought it right to remove Saddam Hussein even if he had known that there were no WMD, he said, but he would obviously have had to "deploy" different arguments. Must we not conclude that the WMD arguments were "deployed" mainly as the best way of selling the war? Blair's comments do not exclude a strong – but mistaken – belief in the existence of WMD even when the invasion was launched. However, given that hundreds of inspections had found no WMD and important evidence had fallen apart, such a belief would have been based on a lack of critical thinking.

How could the issue of – non-existent – WMD mislead the world for more than 10 years? At the end of the Gulf war in 1991 the UN security council ordered Iraq to declare all WMD and destroy them under international supervision. However, Iraq chose to destroy much material without any inspection, giving rise to suspicions that weapons had been squirrelled away. These were nurtured by the frequent Iraqi refusals throughout the 90s to let UN inspectors enter sites and by evasive and erroneous responses to inspectors' inquiries.

What other reason could there have been than to prevent inspectors getting evidence of existing weapons? It is possible that Saddam wanted to create the – false – impression that he still had WMD. What seems more likely to me, however, was a sense of hurt pride, a wish to defy and the knowledge that some of the inspectors worked directly for western intelligence – perhaps even passed information about suitable military targets.

Only in September 2002, when the US had already moved troops to Kuwait, did Iraq say it was to accept the inspection that the UN demanded. By that time a new US national security strategy declared that it could take armed (pre-emptive or preventive) action without UN authorisation; many in the Bush administration saw UN involvement as a potential impediment.

Many are convinced that the American and UK military plans moved on autopilot, and the inspections were a charade. I am sure that many in the Bush team felt that way. It seems likely that British and American leaders expected that UN inspections would again be obstructed or that Iraqi violation of the draconian new resolution 1441 would persuade the security council to authorise military action to remove the regime. For my part, I tended to think of the war preparations rather as a train moving slowly to the front and helping to make Iraq co-operative. If something removed or reduced the weapons issue, the train, I thought, might stop.

For the UK to join the US on an unpredictable UN line was a gamble – and in the end it failed. Inspections did not turn up any "smoking guns" and gradually undermined some of the evidence that had been invoked. Iraq became more co-operative and showed no defiance that could prompt the authorising of armed force. Thus, while the train of war moved on, the UN path pointed less and less to an authorisation of war.

What could the UK have done to avoid this development? It could have made a condition of its participation in the enterprise that the movement of the military train be synchronised with the movement on the UN path. With inspections just starting in the autumn of 2002 the military train should have moved very slowly. We have heard that Karl Rove had said that the autumn of 2003 was the latest time for invasion. Why so fast then in 2002? As the then German foreign minister, Joschka Fischer, said: what was the sense of demanding UN inspections for two and a half years and then let them work only for a few months? Of course, if regime change – and not WMD – was the main aim, the steady speed becomes logical.

The responsibility for launching the war must be judged against the knowledge that the allies had when they actually started it. The UK should have recognised that no smoking gun had been found at any time, and that in the months before the invasion evidence of WMD was beginning to unravel. As we have heard recently: out of 19 Iraqi sites suspected by the UK – and suggested to the UN monitoring, verification and inspection commission for inspection (Unmovic) – 10 were actually inspected, and while "interesting", none turned up any WMD. This warning that sources were not reliable seems to have been ignored. Intelligence organisations seem to have been 100% convinced of the existence of WMD but to have had 0% knowledge where they were. Worse still: the uranium contract between Iraq and Niger that George Bush had given prominence in his 2002 state of the union message was found by the International Atomic Energy Agency to be a forgery.

The absence of convincing evidence of WMD did not stop the train to war. It arrived at the front before the weather got too hot and the soldiers got impatient waiting for action. The factual reports of the IAEA and Unmovic did, however, have the result that a majority on the security council wanted more inspections and were unconvinced about the existence of WMD.

At the end the UK tried desperately to get some kind of authorisation from the security council as a legal basis for armed action – but failed. Confirming the fears of Dick Cheney, President Bush's vice-president, the UN and inspections became an impediment – not to armed action, but to legitimacy.

Unlike the US, the UK and perhaps other members of the alliance were not ready to claim a right to preventive war against Iraq regardless of security council authorisation. In these circumstances they developed and advanced the argument that the war was authorised by the council under a series of earlier resolutions. As Condoleezza Rice put it, the alliance action "upheld the authority of the council". It was irrelevant to this argument that China, France, Germany and Russia explicitly opposed the action and that a majority on the council declined to give the requested green light for the armed action. If hypocrisy is the compliment that virtue pays to vice then strained legal arguments are the compliments that violators of UN rules pay to the UN charter.